The present invention relates to a gate circuit employed in a central control unit of a CATV system. More particularly, the invention relates to such a gate circuit in a central control unit of a type in which unauthorized viewing of designated television channels is blocked by the use of an interference signal superimposed on the television signal to be jammed.
In this type of system, because the interference signal is superimposed on the television signal to be jammed at a distribution unit, there is an advantage over the conventional jamming system, trap system, or addressable terminal system in that it is relatively inexpensive, provides a sufficient jamming effect, and provides a picture signal of good quality (for unjammed or authorized channels) since there are no scrambling signals mixed with authorised channels which have to be removed.
In the type of viewing control system to which the invention pertains, a gate circuit is provided for controlling the flow of the interference signal to a coupler which, when the interference signal is received thereat, adds it to the television signal in the frequency band of the unauthorized channel. Such a gate circuit is shown in the schematic view of FIG. 1.
A group of television signals transmitted from a brancher is amplified by an amplifier 3, and then applied to a distributor 4. One of the outputs of the distributor 4 is applied to the input of a coupler 9, and from there the signal is supplied to a terminal unit in the subscriber's home. A control signal Vt is applied to a jamming generator 5 (implemented, for instance, with a variable frequency oscillator) from a jamming controller 6, the latter receiving a control signal from the control center 1. The jamming generator 5 produces an interference signal fJ at a frequency determined by the value of the signal Vt. The jamming controller outputs a signal Vg which controls the opening and closing of a gate circuit 8. Thus, the flow of the interference signal from the jamming generator, fed via a distributor 7, to the coupler 9 is controlled.
The frequency of the interference signal fJ, which is preferably about 200 KHz from the video carrier of the channel to be jammed, is changed in time sequence in steps of 6 MHz, corresponding to the standard channel spacing. That is, as shown in FIG. 2, during the time period from t1 to t2, the signal fJ is at a level Vt1, changing to a level of Vt2 at time t2 in an extremely short time period .DELTA.t. This process continues until the level of fJ reaches Vt5 in this example, and then returns to Vt1. This repeats cyclically. As shown in FIG. 3, the signal Vg takes two values, a voltage value Vgo rendering the gate 8 open and a value Vgc rendering the gate 8 closed. The resulting frequency spectrum of the interference signal fJ is shown in FIG. 5, ranging between fJ1 and fJ5.
It is assumed that television channel signals at video frequencies fv1, fv3, fv8, and fv11 are to be jammed. To do this, the interference signal is gated on in the intervals when it is at the frequencies fJ1, fJ2, fJ4, and fJ5, which are spaced in frequency by about 200 KHz from fv1, fv3, fv8, and fv11, respectively. For other channels, the interference signal is gated off in the corresponding time periods, thereby greatly attenuating the interference signal in those time periods, as shown in FIG. 4. The frequency spectrum for the interference signal fJ for this case is shown in FIG. 6.
When the frequency of the interference signal fJ is being changed, some energy from this signal unavoidably falls within the band of television channel signals which are not intended to be jammed. To avoid interference with these television signals, as shown in FIG. 7, the gate circuit 8 is opened at each frequency transition time of the interference signal for a short time .alpha. on either side of the center of the transition period. For instance, assuming that fv3 is to be jammed, the gate circuit 8 is closed in the period t3-.alpha. to t3+.alpha.. Accordingly, only components at frequencies fJ1, fJ2, fJ4 and fJ5 are superimposed on the output television signal applied through the coupler 9. The frequency spectrum of the output television signal is shown in FIG. 8.
The structure of the conventional gate circuit is shown in FIG. 9. The interference signal fJ is applied via a terminal 20 to the primary of an input transformer 10, and the control signal Vg is applied via a terminal 21 to a switching voltage generator 19. The input transformer 10 divides the interference signal into positive and negative components at its output secondary windings, which have first terminals connected to diodes 12 and 13, respectively. The attenuation of each of the diodes 12 and 13 to high-frequency signals is determined by the polarity of the anode-cathode voltage across the diode. A constant DC voltage Vc on a terminal 23 is divided by resistors 14 and 15, and the divided voltage is applied to the cathodes of the diodes 12 and 13 via the primary of an output transformer 11. The output of the switching voltage generator 19 is one of two values, Vgc or Vgo, depending upon the value of the signal Vg applied at its input. The output of the switching voltage generator 19 is applied through resistors 16 and 17 to the other terminals of the secondary windings of the transformer 10 and thence to the anodes of the respective diodes 12 and 13. The voltages Vgc and Vgo are determined relative to the DC voltage at the terminal 23 so as to oppositely set the polarity of the DC voltage across the diodes 12 and 13, thus controlling the flow of the television signals between the input transformer 10 and the output transformer 11. That is, when the switching signal generator 19 outputs a voltage of Vgo, the diodes 12 and 13 are reverse biased, and hence the television signal is prevented from being applied to the output transformer 11, while when the switching signal generator 19 outputs a voltage of Vgc, the diodes 12 and 13 are forward biased, and hence the television signal is applied to the output transformer 11.
In FIG. 9, reference numeral 18 indicates a capacitor providing an AC path between the center terminals of the secondary windings of the input transformer 10.
When the circuit of FIG. 9 is used in the viewing control system of FIG. 1, however, a difficulty arises. Specifically, when the gate circuit 8 is in the closed state, the impedance Z at the output of the gate circuit 8 seen by the coupler 9 is that of the distributor 7, which is appropriately matched to that of the transmission lines and other components of the system, and hence no problem arises. However, when the gate circuit 8 is in the open state, the impedance seen by the coupler 9 is infinite; that is, an unmatched impedance is seen by the coupler 9. Therefore, if a channel having a video frequency fv6 is not to be jammed with the corresponding jamming signal fJ3 between times t3 and t4 as indicated in FIGS. 10 and 11, an open impedance is seen by the coupler 9 in the time period of t3 and t4, as depicted in FIG. 12. Accordingly, the signal level of the television signal at the output of the coupler 9 changes as shown in FIG. 13, which has undesirable effects on the quality of the television signal received at the subscriber's terminal unit.